188 2.J 75 [Crosby. 



rejects : viz., (1) thnt which refers these fractures to the operation 

 of magnetic forces, and (2) that which explains them as due to 

 an unexplained shearing force. In other words, fatal objections 

 beset every explanation heretofore proposed, and the problem 

 presented by ordinary or parallel jointing would appear to be still 

 unsolved. The recent discussion of this subject, however, has led 

 me to examine with greater care than ever before an idea which 

 has often been in my mind, viz., that earthquakes are the cause of 

 parallel joint-structure ; and I am persuaded that we have here 

 an explanation which, if not the true one, is at least open to less 

 obvious objections than any that have preceded it. 



An earthquake, it is well known, is a series of spherical, elastic 

 waves originating in a shock or jar at a considerable depth below 

 the surface. The velocity of the earth-waves, when not very high, 

 is determined solely by the elasticity of the medium ; and the 

 normal velocity in solid crystalline rocks, like granite, as shown 

 by the progress of slight tremors and sound-waves, is 10/00 to 

 12,000 feet per second. But observation of many earthquakes, as 

 well as Mallet's experiments, shows that the actual velocity of 

 the spherical wave, in the case of severe earthquakes is often 

 less than one-eighth and rarely exceeds one-fourth of the normal 

 velocity, or 1200 to 1600 feet per second. The enormous differ- 

 ence is satisfactorily explained as due to a difference in wave- 

 height. For, while the rocks, which are at best but very imper- 

 fectly elastic, carry successfully without rupture the low waves, 

 they are broken at every step by the passage of the high waves of 

 the important earthquake. 



This explanation, which I have taken almost verbatim from 

 Le Conte's Elements of Geology, appears to be generally accepted 

 by geologists. ISTow all parts of the earth's crust are affected by 

 earthquakes ; and, during the course of geological time, the num- 

 ber, even of severe shocks, occurring in those areas most nearly 

 free from seismic disturbance must be very considerable. But all 

 important vibrations break the rocks which they traverse ; and 

 hence the conclusion follows irresistibly that all geological forma- 

 tions (except, perhaps, the most recent, which are precisely those 

 freest from joint-structure) must be extensively broken as a con- 

 sequence of the passage of the numerous earthquakes which they 

 have experienced. 



